When hurricane Irene failed to flood Manhattan as predicted over the weekend, many called the evacuation an overreaction to media hype - one local told the New York Times he was "embarrassed." But scientists and civil engineers say the city's response made good sense.

What happened in New York was a success story - not just because the metropolis survived relatively unscathed, but because the city had an evacuation plan in place that was based on up-to-the-minute models of flood behavior. Here's why parts of New York had to empty out, and how city planners created an evacuation plan for the biggest metropolis in America.

The inexact science of predicting storm surges

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As many New Yorkers now know firsthand, storm predictions can be inexact. Thanks to satellite images and weather prediction models, meteorologists could see Irene coming days away. Most regions had days to prepare for the onslaught, and knew what to expect. Except when it came to hurricane intensity.

Georgia Tech [meteorologist Judith] Curry said one of the main problems is that the giant global computer models that do so well forecasting the track require large scale data. The keys to intensity changes are usually too small for big computer models, she said.

[Former hurricane center director Max] Mayfield says what's needed is better real-time, small-scale information, like Doppler radar. NOAA used old propeller planes to take Doppler radar data inside Irene, but the information will be used to design better intensity forecasts in the future, he said.

Why did Irene slow down from a category 3 storm to a bad-but-not-disastrous tropical storm so quickly once it made landfall? Without more data, no one can say for sure.

The point is, until the storm hit, all the very best data suggested it might have the power to flood large parts of Manhattan the way it flooded other cities and towns from Vermont to New Jersey. Though few remember the last time the New York stood in the path of a hurricane, the city still has plenty of data and pictures from 1938's "Long Island Express," a hurricane that ripped through Long Island and flooded huge parts of Manhattan. Based on information from that long-ago disaster, and the many smaller disasters since, the city already had detailed plans for an evacuation, complete with models of which regions of the city would be hardest hit.

The New York bight and the creation of the city's evacuation plan

The biggest danger to an urban area like New York, other than strong winds, are storm surges, massive domes of ocean water that can make landfall hours before the storm itself. In New York, this is an especially big problem due to an underwater formation called a "bight" off the coast. Beneath the Atlantic waters, there is an enormous triangular bite taken out of the shoreline leading into the mouth of the Hudson River. This can amplify any incoming storm waves, creating an especially big surge tearing through Manhattan on the back of its encircling river.

GIS use geospatial methods to analyze, interpret, and present geospatial information. For the case of hurricane evacuation planning, geospatial hurricane and hurricane surge predictions made using physics-based computational models would be added as input in the GIS to evaluate their impact in a geographic area of interest.

Like most regions in the United States, New York City tries to figure out how the city will be affected by floods using computer modeling software called Sea, Lake, and Overland Surges from Hurricanes (yes, that's SLOSH for short). SLOSH creates maps of storm flooding in a given area based on the severity of the storm, water levels, and elevation, along with some other factors.

New York City's hurricane evacuation plan designates three separate zones, called simply A, B, and C. Zone A corresponds to the lowest-lying regions of the city most susceptible to hurricanes and storm surges. Zones B and C correspond to areas that will be flooded by higher category hurricanes. In preparation for Irene, city officials issued evacuation orders to people in Zone A and parts of Zone B. This fit with predictions that the storm was diminishing in intensity, but might still be category 1 when it hit Manhattan.

University of Western Ontario civil engineer Slobodan Simonovic, who has worked on flood models both in Canada and the US, told io9 that New York City's response was ideal, given the circumstances. "From the standpoint of disaster preparedness, the disaster managers have only the forecast to work with," he said. "Better prepared is a much better option than not being prepared." He added that New York's evacuation contrasted favorably with New Orleans' preparation for hurricane Katrina, which stands as an example of how bad management can worsen a disaster's impact.

Simonovic's sentiments were echoed by Chris Gilbried, who spoke to io9 on behalf of NYC's Office of Emergency Management:

Hurricanes are unpredictable. You must make predictions based on information that is available.

Take Zone A, for example. Right up until the day before Irene made landfall, forecasters were predicting 4-8 feet of storm surge in Zone A, and the vast majority of people killed in hurricanes are killed by storm surges.

We also had to evacuate parts of Zone B on Rockaway, which is a barrier island with two bridges. The transportation commissioner said that if windspeeds reached over 50 mph, they would have to evacuate Rockaway; we didn't want people to be cut off.

Again, the lesson learned from Katrina loomed over Manhattan. Storm surges could be a huge danger, felling bridges (or levees), and there was no choice but to prepare for the worst. People were forced to evacuate.

Mandatory evacuation blues

As civil engineer Simonovic explained, the hardest part of preparing for any disaster is often the human factor. This is especially true when you're dealing with a population the size of New York City's, where there is almost no living memory of having dealt with hurricane and flood disasters before.

In a study published in 2005, 1500 residents of New York City were interviewed to gauge their responses to questions concerning hurricane evacuation. The report explains:

The population has too little experience responding to hurricane threats to have developed a response culture. There are no relatively recent hurricane evacuations in the city to learn from.

Simonovic amplified:

Experience is something that affects how people respond to disasters . . . it affects our assessment of risk. If I lived through a disaster then I can measure against something. I know what that [hurricane] wind potentially could mean. These fine psychological factors affect response. I don't think too many people in New York have experience with flooding. This is a new type of situation for them, and this may lead to misjudgment in what's the right way to respond.

By contrast, he noted, people in Japan have a national disaster prevention day every year on September 1. That day, the entire nation — from the Prime Minister to schoolchildren — goes through a disaster drill, practicing for when the next storm or earthquake will hit. Everyone in Japan knows where to go if there's a flood or quake, and where to wait for help.

And yet most New Yorkers don't even know where the evacuation zones in their city are, let alone where to go in an emergency. The 2005 report states:

The biggest obstruction to having an efficacious evacuation response in New York is lack of familiarity with the evacuation zones, their boundaries, and the rationale behind them. At least 75% of the residents surveyed said they were not familiar at all with the evacuation zones. Among people who said they knew whether they lived in an area needing to evacuate, the great majority were unable to correctly identify which zone they lived in.

It's worth mentioning that the 2005 NYC hurricane evacuation study acknowledges mass evacuation is far from the best answer; it's a last resort, and city officials will evacuate as few people as possible over as much time as preparation will allow. Emergency officials gain nothing from over-evacuating — the city becomes congested, and the truly endangered areas may not get enough resources to cope with the disaster.

The disaster next time

Given that most New Yorkers had almost no idea how to cope with a major hurricane until last weekend, the city came through pretty well. In fact, the evacuations will no doubt improve the city's next disaster response, since most New Yorkers are now painfully aware of whether they're in Zone A, B, or C. In the absence of disaster preparedness day, Irene Day will have to do. And it's a good thing, too, as the New Yorker's environment journalist Elizabeth Kolbert points out:

Are more events like Irene what you would expect in a warming world? Here the answer is a straightforward "yes." In fact, experts have been warning for years that New York will become increasingly vulnerable to storm surges and flooding as the planet heats up. In 2009, the New York City Panel on Climate Change, appointed by Mayor Bloomberg, concluded that, as a result of global warming, "more frequent and enhanced coastal flooding" was "very likely" and that "shortened 100-year flood recurrence period" was also "very likely."

Of course, some models suggest that global warming might actually dampen storms and lessen the threat of hurricanes in New York. Very soon we'll find out which model tells us more about what kinds of disaster to prepare for.

Either way, it's important to remember that no evacuation is ever perfect. In New York, there were many stories of people hunkering down in Zone A and ignoring pleas to get out from the city's disaster management groups. Simonovic says that in Canada, people who refuse to evacuate can be forcibly removed from their homes by police. Police will not do that in the United States, so a percentage of people get left behind at their peril.

Still, when disasters are bad, it turns out that the vast majority of us will make the decision to comply with a mandatory evacuation order. University of Nevada civil engineering professor Sajjad Ahmad, an expert in disaster management, wrote to io9 in an email:

Compliance rate in disasters such as nuclear or chemical hazards is typically very high (in high 90s). For events such as hurricanes it also depends on how severe the event is; a category 4 hurricane will get much better compliance compared to a category 1 . Compliance rates vary between 20% to 90%.

Let's hope we never have to witness a disaster that inspires 90% compliance in a city like New York. But in case we do, it's good to have a plan.

As civil engineer Simonovic says:

In New York, it was was a fortunate thing that you had 4 days to plan - there are types of disasters where you have only 15 minutes. That was why, in my opinion, it was properly done. Those four days gave them time to assess the potential impact and come up with an evacuation plan. And then two days before the possible storm surge they provided evacuation orders. My sense is that this was properly done despite the fact that people may be critical.

Though the New York evacuation may seem overcautious in retrospect, hurricane Irene could have been a lot worse than it was. During this storm, the city showed that it was ready and able to survive something a lot bigger than what came its way. And that kind of scenario is always a win.